American Military History, Volume 2 by Richard W. Stewart

Even as the Germans launched their June and July offensives, Gen- eral Foch had been looking for an opportunity to strike a counterblow. The Marne salient presented an excellent prospect: the salient was in- herently weak as the German forces relied on a single railroad through Soissons for the majority of their supplies. The Germans had failed to improve the situation with their June offensive. In mid-June Foch directed Petain to begin making plans for an attack against Soissons; Petain and his commanders completed the plans by the end of June. After French intelligence had warned him of the German attack east of Chateau-Thierry that would begin on July 15, Foch set the date for his counterattack as the eighteenth. Consequently, as the Germans were attacking on the eastern flank of the salient, the Allies would be attacking against their exposed western flank.

The Allied attack plan called for two French armies to attack on July 18 toward Braine on the Vesle River. In the north, the French Tenth Army would conduct the main attack between the Aisne and the Ourcq Rivers; in the south, the French Sixth Army would attack between the Ourcq and the Marne. Their mission was to cut the German lines of communications in the salient. The French Fifth and Ninth Armies on the eastern flank would join the attack after defeating the German of- fensive. Foch expected the reduction of the Marne salient to follow.